Recently, the amount of heat generated by, for instance IC devices increases steadily as the progression of the miniaturization and speedup of electronic machinery and tools and accordingly, it has been required for a heat-radiating member used around the surroundings of heat-generating parts of the devices to have further improved heat-radiation characteristics. In response to this, there has been under study the use of a resin or rubber filled with alumina powder as one of such heat-radiating members.
Alumina is present in a variety of crystalline states such as α, β, δ, γ and θ-crystalline states, but α-alumina can suitably be used as a material for a heat-radiating member among others, since it has the highest heat conduction. However, α-alumina powder is in general in the forms free of any crushed shapes or cut edges (or in the form of edge-free shapes or round shapes). For this reason, in the existing circumstances, it cannot be incorporated into resins or rubber at a high density or in a large amount and thus one cannot make the most use of the high heat conduction of α-alumina. In addition, a problem also arises such that these α-alumina powdery products may severely be worn out, for instance, the machinery and tools used in the blending thereof with resins or the like such as kneaders and rolls and pumps used for transporting a composition including, for instance, a resin as well as a mold or the like used for molding the composition.
For this reason, there have variously been investigated the use of spherical alumina powder.
In the meantime, spherical alumina powder is prepared according to a technique fundamentally comprising a step of melting a raw material of alumina through heating the same with a flame and the resulting alumina powder is in crystalline states such as δ, θ, γ, and β-crystalline state, which are inferior in the heat conduction to α-state. For instance, Patent Document 1 (specified below) discloses a method for preparing spherical α-alumina powder, but the results of the supplementary examinations of the method carried out by the inventors of this invention indicate that the content of α-alumina in the resulting product is 85% and a problem thus arises such that the method provides only a product having an average sphericity on the order of at highest 0.91 as expressed in terms of the evaluated value of the sphericity as will be described later. In addition, Patent Document 2 (specified below) discloses a method for preparing pseudo-spherical particles through the agglomeration of fine α-alumina particles, but the method of this literature simply provides, in the existing state, α-alumina powder whose average sphericity is at highest about 0.80.    Patent Document 1: JP-A-2001-019425;    Patent Document 2: JP-A-9-086924.